class Watcher {
  constructor (vm, key, cb) {
    this.vm = vm
    this.key = key
    this.cb = cb
    Dep.target = this
    this.oldValue = vm[key]
    Dep.target = null
  }
  update () {
    let newVal = this.vm[this.key]
    if (this.oldValue === newVal) return
    this.cb(newVal)
  }
}